1. Field of the Invention
This invention relates to improvements in the electrical conductivity of cured silicone elastomers produced from silicone elastomer compositions.
2. Description of the Prior Art
Compositions comprising elastomeric materials filled with electrically conducting particles are well known in the art. The particles have been described as metallic particles, carbon black, colloidal graphite particles, and metallic coated particles. Many types of elastomeric materials have been described including silicone elastomers. Japanese Patent Publication No. 24,300 published July 23, 1976 by Yoshita and Takeachi describes a composition which can be vulcanized under standard pressure consisting of diorganopolysiloxane, 0.1 to 100 parts of carbon fiber, and 3 to 10 parts of 2-4-dichlorobenzoylperoxide. The patent teaches that carbon blacks such as acetylene black are not allowed. The patent further teaches that a procedure employing a platinum catalyst for vulcanization is not satisfactory. Ellis, in U.S. Pat. No. 4,064,074, claims a process for the preparation of an electrically conductive coating composition comprising a particulate component and a vehicle. Among many suggested vehicles or binders is silicones, further defined as silicone resin or silicone rubber. The particulate component is a blend of 60 to 90 percent graphite, 1.5 to 20 percent manganese dioxide, and 0.5 to 20 percent zinc oxide. The electrically conductive coating composition is used in the manner of a paint.
In U.S. Pat. No. 3,284,751 Barker et al. describe an ignition cable for use with automotive vehicles comprising a flexible resistive conductor for nonconducting fibers coated with conducting material of discrete particles, a conductive rubber coating bonding together the fibers of said conductor, and other additional layers. The composition of the instant invention is an improvement over presently known commercially conductive rubber coatings. These coatings, as described by Barker, et al. are solvent solutions which are applied by dipping, drying, and curing. Such a process requires large amounts of energy for the drying step. The solvent also causes a problem in its proper ecological disposal. A preferred embodiment of the instant invention uses no solvent, thereby lowering the energy requirement and also eliminating the disposal problem.
The Barker et al. process entails the coating of a layer of conducting substance over said rubber coating to act as a release agent for subsequent layers. This coating, consisting of a colloidal solution of graphite in alcohol, must be dipped and dryed. An electrically conductive member produced according to the instant invention does not require such a coating when used in an ignition cable since the surface of the electrically conductive member is a good release surface for insulation applied over it.
Barker et al. further describes the construction of a prior art nonmetallic conductor. Individual threadlike filaments, of cotton, rayon, linen, polyester, or glass, or mixtures of the various filaments are impregnated with conductive material, such as graphite. The individual threads are immersed in a colloidal solution of graphite in alcohol so as to completely impregnate the individual fibers of the threads. Upon issuing from the graphite solution, the threads are gathered together in a suitable mechanical means and carried into a drying oven as a group so that they are dried together in contact with each other which deposits the graphite as discrete particles on the threads. The resistance of the final conductor is in part determined by the number of threads used and in part by subsequent conductor components.
The construction of the electrical conductive member of the instant invention can be much simpler and cheaper than that taught by Barker et al. The electrical conductivity of the composition of the instant invention can be high enough to allow the use of electrically nonconducting filaments. The above described step by Barker et al. for making the filaments conductive can be completely eliminated from the process.